The invention relates to a blown film comprising a polypropylene composition. In particular, the invention relates to polypropylene blown films having excellent optical properties, as well as mechanical properties.
Polypropylene for films is today almost exclusively used for cast films. In the cast film process a very quick cooling of the melt is achieved with a chill roll, in order to utilise the potentially good optical and mechanical properties of polypropylene for film applications.
Until now, polypropylene could be used in the blown film process only when it was possible to use water contact cooling in order to achieve the same quick cooling as in the cast film process.
In a conventional blown film process using air cooling, the polypropylene melt and the films produced from it are only slowly cooled. This results in insufficient optical and mechanical parameters because the slow cooling process causes an uncontrolled growth of the crystal- and spherolitic structure. Polypropylene blown films produced with air cooling are both cloudy and brittle.
It has been tried to improve the problem of cloudiness and brittleness of polypropylene blown films by using a combination of propylene random copolymer with either inorganic nucleating agent or sorbitol-based nucleating agents. Improvements of optical and mechanical parameters were only possible to a very limited extent.
It is therefore the object of the invention to provide a polypropylene blown film having good optical parameters, comparable to those obtained with a cast film, together with a well balanced stiffness/drawability ratio.